In many systems it is important to obtain an accurate measure of the amplitude of incoming signals. This is especially true in test systems, for example, those employed to evaluate transmission facility characteristics.
Of particular interest are measurement circuits employed in responder apparatus used to evaluate characteristics of telecommunications transmission facilities. Prior known responder apparatus are employed to obtain measurements of, for example, loss, low level loss and various noise characteristics of telephone trunk facilities. To this end, responder receiver units employ a measurement circuit which includes an amplitude conversion circuit for obtaining a measure of the amplitude of incoming signals and for facilitating transmission of the measurement result to remote locations. Typically a test signal for the particular test being run is transmitted from a first location at one end of a transmission facility under evaluation and is received at a second location. The amplitude of the received signal is measured, converted into digital form by employing an amplitude to pulse width conversion and is utilized as desired. Usually, the measurement result is transmitted to the first location.
In order to obtain accurate measurements of the characteristics being evaluated, the prior known responder measurement circuits used extremely precise circuit components and complex circuit designs which required precise manual adjustment to compensate for nonlinearities of the circuit components in order to insure accurate amplitude measurements and conversion. The manual adjustment of the prior measurement circuits to obtain reasonable precision of measurement is usually done on installation of the equipment and periodically thereafter. Indeed, use of precision circuit components, complex circuit designs and manual adjustment of the circuits is undesirable from an economic standpoint. Such adjustment still does not compensate for ongoing variations caused by component aging and environmental changes.
In one known computer controlled test system, for example as disclosed in U.S. Pat. No. 3,842,247 issued to T. C. Anderson on Oct. 15, 1974, an attempt is made toward minimizing the affects of test system characteristics by employing a so-called calibration run prior to a test run. The calibration run, however, only compensates at a single amplitude level and still requires the use of a precision test signal generator and a precision measurement circuit in the receiver. Thus, T. C. Anderson, in his prior computer controlled arrangement, is not concerned with compensating amplitude measurement results for possible errors over an entire amplitude range.